3/10x+7x-1/4x=21.15 | 4(2x-1)=3(x+4) | (19x+2/5) 除2=4 | x+1/4(1-2/7x)=8/7
6/20x+140/20x-5/20x=21.15 | 8x-4=3x+12 | 19x+2/5=8 | x+1/4-1/14x=8/7
141/20x=21.15 | 5x=16 | 19x=38/5 | 13/14x=25/28
x=3 | x=16/5 | x=2/5 | x=50/13
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5(x-4)+3(9-x)=12+7(7-7x)
5x-20+27-3x=12+49-49x
51x=54
x=54/51